1. Field of the Invention
This invention relates to a float of synthetic resin for a carburetor which is used to supply fuel to an engine of a vehicle, and in particular, to a float usually called a double float which has two float parts.
2. Description of Related Art
In recent years, according to the need for compactness of the carburetor, a float chamber has been designed to have the smallest possible size. Thus, in most cases, a partition wall between the float chamber and a suction tube assumes a special shape, and a jet nozzle for fuel is located in the vicinity of the middle of the float chamber. For a float placed in the float chamber of such a narrow, complicated interior shape, a double float has proven useful.
The double float of prior art is generally constructed with a base having a mounting portion for swingably mounting the float in the float chamber, an arm portion connected to the base, and two float parts connected the arm portion so that they are located opposite to each other at different positions of the arm portion. Such arm portions are available in various shapes; typically, those in which the arm portion is configured into a roughly straight form, those in which it is bent at a predetermined angle from a connection of the arm portion with the base and is configured to extend roughly straight to the float parts, and those in which it is configured into a curved form.
Such a double float for carburetors must be constructed so that it can fit into and operate within in a narrow float chamber, thus requiring a high degree of dimensional accuracy. For example, in the case of an extremely small carburetor used in a motorcycle, tie carburetor must be manufactured such that the float is placed to keep the space between the float and the wall of the float chamber to about 1 mm and is operated in this state.
In recent years, on the other hand, the development of various parts made of synthetic resin has taken place. The major reasons for this are that lightweight parts can be made and parts of complicated shape can be mass-produced at low cost. Since the double floats for small-sized carburetors must also be manufactured so as to be light-weight and to accommodate the special shape inside the float chamber, most of them have come to be made of synthetic resin.
However, it is difficult for a double float requiring a high degree of dimensional accuracy to be constructed of synthetic resin. For this respect, two prior art examples having typical structures will be explained below in reference to drawings.
First, in FIG. 1, a description is given of the prior art example of a double float assembly with a straight arm. FIG. 1 shows the float mounted in the float chamber, viewed from above.
A base 1 of the float includes a mounting portion 1a and a control portion 1b. The mounting portion 1a is cylindrical and receives a shaft disposed in the float chamber which is rotatably fitted into a center bore 1a1 to thereby mount the float in the float chamber. In this way, the float can be swung in accordance with the internal fuel capacity of the float chamber.
The control portion 1b connected to the mounting portion 1a has the shape of a plate and is designed so that when the internal fuel capacity of the float chamber reaches a predetermined amount, the lower end of a known needle valve is pushed upward to control the supply of the fuel into the float chamber. When the amount of fuel in the float chamber is decreased, the needle valve is lowered to keep the fuel in the float chamber to the predetermined amount. Usually, in order to surely lower the needle valve, the control portion 1b is removably mounted to the lower end of the needle valve.
The base 1 is joined to an arm portion 2 in the vicinity of the middle in a longitudinal direction of the arm portion 2 and assumes such a shape that the arm portion 2 extends straight from both sides of the base 1. Some of the prior art double floats, however, are such that the base 1 is not joined in the vicinity of the middle of the arm portion 2 but is shifted to the right or left therefrom. The present invention is also applicable to the floats of such shape.
In FIG. 1, the arm portion 2 has the shape of a plate in which a dimension in a direction perpendicular to the plane of this figure is smaller than that in a vertical direction parallel to the plane of the figure, and the former dimension is constant. The arm portion 2 is provided with a projection 2a on the right hand side of the base 1. The projection 2a is adapted to abut a stopper of the float chamber when the amount of fuel in the float chamber is excessively decreased.
Float parts 3 and 4 are connected to both ends of the arm portion 2 and are arranged opposite to each other. As is well known, covers 3a and 4a are attached to the float parts 3 and 4 by means of adhesion or welding so that the float parts 3 and 4 are hollow. Thus, this prior art double float, with the exception of the covers 3a and 4a, is integrally molded by using a set of dies.
In FIG. 2. reference is made to the prior art example of a double float assembly in which the arm portion is curved. A base 5 of this float is provided with a cylindrical mounting portion 5a. The float is swingably mounted in the float chamber on a shaft that is rotatably fitted into a bore 5a1.
Furthermore, the base 5 is equipped with a needle valve and a control portion 5b for controlling the needle valve. Since a specific shape of the control portion 5b and the mounting of the needle valve are the same as in one of the embodiments which will be described later, their detailed explanations are omitted here.
Tile base 5 is joined to a curved arm portion 6 in the vicinity of the middle in a longitudinal direction of the arm portion 6. The arm portion 6, like the arm portion 2 shown in FIG. 1, has the shape of a plate which is constant in thickness (in a direction normal to the plane of the figure), and float parts 7 and 8 are connected opposite to each other to both ends of the arm portion 6. The float parts 7 and 8 are hollow, and their covers 7a and 8a are mounted on the front sides of the float parts 7 and 8 with respect to the figure.
The floats constructed as mentioned above, because of their structures, cannot be easily molded and fabricated. The major problem in manufacturing is that it is difficult to shape the arm portion into a predetermined form. Specifically, for the prior art example of FIG. 1, it is difficult to maintain the straightness of the arm portion 2, and in that of FIG. 2, it is also difficult to shape the arm portion 6 into a predetermined curved form. Unless the straightness and the curved form are accurate, the float parts will come in contact with the wall of the float chamber, and cannot function properly.
As is well known, the melted material of synthetic resin, when cooled, is crystallized and solidified, and in general, its shrinkage rate increases with increasing time for solidification. In the context of a float made of synthetic resin, when the melted material is injected into a metallic mold and placed under a cooling process, an internal region of the injected material cools more slowly than an external region. Since the amount of internal shrinkage in the arm portion is thus larger than the amount of external shrinkage, the float parts are inclined inwardly as indicated by arrows in FIGS. 1 and 2, and the float fails to maintain a predetermined shape.
Thus, where the double float of this type is manufactured, special fabrication techniques, such as a temperature control in the cooling process of the metallic mold and special selection of material, have been used, and other provisions have been made for preventing the deformation of the arm portion. Consequently, fabrication costs have become high and cannot be easily reduced.